Conventionally, a belt transmission has been employed as a transmission for vehicles such as two-wheelers, three- or four-wheeled working vehicles, three- or four-wheeled leisure vehicles, or snow mobiles because the structure is simple, a gear-change is unnecessary and shock is not caused when changing the gear.
The belt transmission has a structure in which a whole belt transmission mechanism comprising a variable pitch diameter type drive pulley driven by a drive shaft of an engine and a variable pitch diameter type driven pulley driven by the drive pulley through a drive belt is isolated from the environment by a housing in such a manner that sand, dust and the like do not get into the contact faces of the drive belt and the pulleys.
Air is sucked into the housing through an intake duct, and is exhausted from an exhaust duct provided at a position on the housing apart from the intake duct so that the inside of the housing is forcedly cooled by the air. Thus, friction heat and other heat generated between the drive belt and the pulleys are exhausted to the outside.
In order to efficiently cool the inside of the housing by the air, a cooling fan is integrally provided on the drive pulley to forcedly suck the air from the intake duct, and the cooling fan is also provided on the driven pulley to forcedly exhaust the air in the housing to the outside. In the belt transmission, it is necessary to efficiently cool the inside of the housing by the air under low speed/high load conditions such as hilly terrains or the case where heavy packages are loaded as well as high speed conditions. In order to efficiently perform air-cooling under the low speed conditions as well as the high speed conditions, accordingly, the cooling fan is provided on the drive pulley and the driven pulley as described above in such a manner that the air is forcedly sucked and exhausted. The prior art of this kind has been disclosed in Japanese Unexamined Utility Model Publication No. 58-96157.
However, if the cooling fan is provided on the driven pulley as described above, a certain amount of power of the engine is consumed for the cooling fan. Consequently, the engine power applied to drive a vehicle is reduced. More specifically, in case of a leisure vehicle on which an engine of about 20 horsepower is mounted and which runs at a maximum speed of about 40 to 60 km/h, about 3 to 4 horsepower is sometimes consumed by the cooling fan of the driven pulley. In other words, runability is deteriorated for the power consumed by the cooling fan of the driven pulley.